To this group are related the special suction-type formations 

 of the attaching discs among representatives of Tetraonchoididae (Fig. 

 29). They are arranged by two pairs on the dorsal surface of the posterior 

 part of the disc so that the first pair is located almost in the middle of the 

 longitudinal axis of the discs, symmetrically along the sides of the longi- 

 tudinal axis; and the second pair lies in the same fashion, but nearer the 

 posterior end of the disc. These suction-type formations are in the shape 

 of little round pillows which stand out on the surface of the disc, each one 

 of them bears a very weakly noticeable chitinous , sickle -type plate. 



The fourth, that is the group of attaching clamps, is charac- 

 teristic of the highest types of monogenetic trematodes. The attaching 

 clamps represent a complex system which arises from a muscular sucker 

 and the powerfully developed chitinous parts; with this on the one hand we 

 can observe forms in which the muscular part predominates, and on the 

 other hand forms with a preponderance of chitinous formations. In the 

 latter case we cannot even mention the word sucker, because this apparatus p. 32 

 functions not as a suction mechanism but as a pinching mechanism. One 

 can observe numerous transitions between the extreme forms which were 

 mentioned. Attaching valves among Hexostomatidae have (Fig. 37) the 

 most primitive structures which represent almost a typical sucker with 



_. 



It must be noted that the morphological primitiveness of the clamps of 

 Hexostomatidae appears to be a result of secondary simplification; on 

 very careful analysis this primitiveness is only illusory (see page -^Z,/). 



three chitinous parts, of which one is X-shaped, and lies in the middle in 

 the special muscular partition which separates the sucker (clamps, nobis) 

 into right and left halves and two others of irregular shape are located on 

 the right and left edges. If you can imagine now that the sucker (clamp, 

 nobis) of such a type will be constructed along the cross axis, then we will 

 obtain the following morphological stage and actually the first real attaching 

 clamp, principally distinguishing itself from the suckers in having ventral 

 and dorsal clamp halves or valves (Fig, 38). 



The subsequent complication (or evolution, nobis) proceeds 

 along the lines of an increase in the number and sizes of chitinous parts. 

 In the typical attaching clamp (Fig. 39) we can distinguish two parts corre- 

 sponding in origin to the right and left halves of the initial suction type; at 

 the same time, a greater part of the chitinous arnnature of one half is 

 larger and of a somewhat different shape than that of the other. We can 

 consider as the rriost widespread type of clamp the type with its chitinous 

 parts situated in the following way: The largest band (Bychowsky uses 

 plate in place of band throughout, western workers employ the word 

 sclerite, nobis) lies along the longitudinal axis, that is across the clamp. 



25 



